Caley M J, Boutin S, Moses R A
Department of Zoology, University of Guelph, N1G 2W1, Guelph, Ontario, Canada.
Oecologia. 1988 Jan;74(4):501-506. doi: 10.1007/BF00380046.
Sex ratios of a population and of litters were sampled in muskrats in Ontario, Canada. Sex ratios of litters sampled from nests were male biased (54% male). Until weaning, no differential costs of producing and rearing male and female young were identified that could account for this greater production of males. Following weaning, however, male-biased dispersal of juveniles from their natal site and more frequent acquisition by females of these sites as breeding sites the following year suggested a greater investment by adult females in female young. Therefore, competition between female siblings for the acquisition of their natal site may be sufficient to result in the greater production of males. In addition, the simultaneous occupation of, and competition between, siblings and parents for the resources of the natal home range may not be necessary for local resource competition to result in a greater production of the dispersing sex. Greater-than-expected binomial variance in sex ratios of litters suggested that adjustment of sex-ratios occurred. However, we were unable to associate the adjustment of litter sex ratios with changes in maternal condition. The greater production of males and the predominance of monogamous associations between adults in this population may have lead to slightly greater variation in male fitness than female fitness. Therefore, a female in better-than-average condition may have benefited by producing more males. Similarly, a lower cost of producing dispersing males may allow nutritionally-stressed females to reduce their total expenditure on offspring by producing more males. Because these experiments were non-manipulative, maternal condition may not have varied sufficiently during this study to detect adjustments of litter sex ratios resulting from either of the above mechanisms acting separately, but the combined effects of small differences in matermal condition and selective pressures operating in the same direction may have resulted in the observed deviation from the binomial.
在加拿大安大略省的麝鼠种群中对窝仔的性别比例进行了抽样。从巢穴中抽取的窝仔性别比例偏向雄性（雄性占54%）。在断奶前，未发现生产和养育雄性与雌性幼崽存在差异成本，这可以解释雄性数量较多的情况。然而，断奶后，幼崽从出生地偏向雄性的扩散以及次年雌性更频繁地将这些地点作为繁殖地获取，这表明成年雌性对雌性幼崽的投资更大。因此，雌性同胞之间为获取出生地的竞争可能足以导致雄性数量较多。此外，同胞和父母同时占据出生地的活动范围并竞争其资源，对于局部资源竞争导致扩散性别的数量较多来说可能并非必要条件。窝仔性别比例中大于预期的二项式方差表明发生了性别比例的调整。然而，我们无法将窝仔性别比例的调整与母体状况的变化联系起来。该种群中雄性数量较多以及成年个体之间一夫一妻制关联占主导地位，可能导致雄性适合度的变化略大于雌性适合度。因此，状况优于平均水平的雌性可能通过生育更多雄性而受益。同样，生产扩散雄性的成本较低可能使营养压力较大的雌性通过生育更多雄性来减少其在后代上的总支出。由于这些实验是非操纵性的，在本研究期间母体状况可能没有足够的变化来检测由上述任何一种机制单独作用导致的窝仔性别比例调整，但母体状况的微小差异和同向作用的选择压力的综合影响可能导致了观察到的偏离二项式分布的情况。
